Biocompatible bonding method and electronics package suitable for implantation

ABSTRACT

The invention is directed to a method of bonding a hermetically sealed electronics package to an electrode or a flexible circuit and the resulting electronics package, that is suitable for implantation in living tissue, such as for a retinal or cortical electrode array to enable restoration of sight to certain non-sighted individuals. The hermetically sealed electronics package is directly bonded to the flex circuit or electrode by electroplating a biocompatible material, such as platinum or gold, effectively forming a plated rivet-shaped connection, which bonds the flex circuit to the electronics package. The resulting electronic device is biocompatible and is suitable for long-term implantation in living tissue.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional application of U.S. patentapplication Ser. No. 10/236,396, filed Sep. 6, 2002, entitled“Biocompatible Bonding Method and Electronics Package Suitable forImplantation,” the disclosure of which is incorporated herein byreference and which is a continuation-in-part of U.S. patent applicationSer. No. 10/174,349, filed on Jun. 17, 2002, entitled “BiocompatibleBonding Method and Electronics Package Suitable for Implantation,” thedisclosure of which is incorporated herein by reference, and whichclaims benefit of U.S. Provisional Application No. 60/372,062, filed onApr. 11, 2002, entitled “Platinum Deposition for Electrodes,” thedisclosure of which is incorporated herein by reference.

The application claims benefit of U.S. patent application Ser. No.10/226,976, filed on Aug. 23, 2002, now U.S. Pat. No. 6,794,533,entitled “Platinum Electrode and Method for Manufacturing the Same,” thedisclosure of which is incorporated herein by reference, and whichclaims benefit of U.S. Provisional Application No. 60/372,062, filed onApr. 11, 2002, entitled “Platinum Deposition for Electrodes,” thedisclosure of which is incorporated herein by reference.

GOVERNMENT RIGHTS NOTICE

This invention was made with government support under grant No.R24EY12893-01. The government has certain rights in the invention.

FIELD OF THE INVENTION

This invention relates to an electrode array or flexible circuit,electronics package and a method of bonding a flexible circuit orelectrode array to an integrated circuit or electronics package.

BACKGROUND OF THE INVENTION

Arrays of electrodes for neural stimulation are commonly used for avariety of purposes. Some examples include U.S. Pat. No. 3,699,970 toBrindley, which describes an array of cortical electrodes for visualstimulation. Each electrode is attached to a separate inductive coil forsignal and power. U.S. Pat. No. 4,573,481 to Bullara describes a helicalelectrode to be wrapped around an individual nerve fiber. U.S. Pat. No.4,837,049 to Byers describes spike electrodes for neural stimulation.Each spike electrode pierces neural tissue for better electricalcontact. U.S. Pat. No. 5,215,088 to Norman describes an array of spikeelectrodes for cortical stimulation. U.S. Pat. No. 5,109,844 to de Juandescribes a flat electrode array placed against the retina for visualstimulation. U.S. Pat. No. 5,935,155 to Humayun describes a retinalprosthesis for use with a flat retinal array.

Packaging of a biomedical device intended for implantation in the eye,and more specifically for physical contact with the retina, presents aunique interconnection challenge. The consistency of the retina iscomparable to that of wet tissue paper and the biological media insidethe eye is a corrosive saline liquid environment.

Thus, the device to be placed against the retina, in addition to beingcomprised of biocompatible, electrochemically stable materials, mustappropriately conform to the curvature of the eye, being sufficientlyflexible and gentle in contact with the retina to avoid tissue damage,as discussed in Andreas Schneider, Thomas Stieglitz, Werner Haberer,Hansjorg Beutel, and J.-Uwe Meyer, “Flexible Interconnects forBiomedical Microsystems Assembly, IMAPS Conference, Jan. 31, 2001. It isalso desirable that this device, an electrode array, provides a maximumdensity of stimulation electrodes. A commonly accepted design for anelectrode array is a very thin, flexible conductor cable. It is possibleto fabricate a suitable electrode array using discrete wires, but withthis approach, a high number of stimulation electrodes cannot beachieved without sacrificing cable flexibility (to a maximum of about 16electrodes).

A lithographically fabricated thin film flex circuit electrode arrayovercomes such limitations. A thin film flex circuit electrode array canbe made as thin as 10 um (0.0004 inches) while accommodating about 60electrodes in a single circuit routing layer. The flex circuit electrodearray is essentially a passive conductor ribbon that is an array ofelectrode pads, on one end, that contact the retina and on the other endan array of bond pads that must individually mate electrically andmechanically to the electrical contacts of a hermetically sealedelectronics package. These contacts may emerge on the outside of thehermetic package as an array of protruding pins or as vias flush to apackage surface. A suitable interconnection method must not only serveas the interface between the two components, but must also provideelectrical insulation between neighboring pathways and mechanicalfastening between the two components.

Many methods exist in the electronics industry for attaching anintegrated circuit to a flexible circuit. Commonly used methods includewirebonding, anisotropic-conductive films, and “flip-chip” bumping.However, none of these methods results in a biocompatible connection.Common materials used in these connections are tin-lead solder, indiumand gold. Each of these materials has limitations on its use as animplant. Lead is a known neurotoxin. Indium corrodes when placed in asaline environment. Gold, although relatively inert and biocompatible,migrates in a saline solution, when electric current is passed throughit, resulting in unreliable connections.

In many implantable devices, the package contacts are feedthrough pinsto which discrete wires are welded and subsequently encapsulated withpolymer materials. Such is the case in heart pacemaker and cochlearimplant devices. Flexible circuits are not commonly used, if at all, asexternal components of proven implant designs. The inventor is unawareof prior art describing the welding of contacts to flex circuits.

Attachment by gold ball bumping has been demonstrated by the Fraunhofergroup (see Hansjoerg Beutel, Thomas Stieglitz, Joerg Uwe Meyer,“Versatile ‘Microflex’-Based Interconnection Technique,” Proc. SPIE Confon Smart Electronics and MEMS, San Diego, Calif., March 1998, vol 3328,pp 174-82) to rivet a flex circuit onto an integrated circuit. A robustbond can be achieved in this way. However, encapsulation provesdifficult to effectively implement with this method. Because the gapbetween the chip and the flex circuit is not uniform, under fill withepoxy is not practical. Thus, electrical insulation cannot be achievedwith conventional under fill technology. Further, as briefly discussedearlier, gold, while biocompatible, is not completely stable under theconditions present in an implant device since it “dissolves” byelectromigration when implanted in living tissue and subject to anelectric current (see M. Pourbaix, Atlas of Electrochemical Equilibriain Aqueous Solutions, National Association of Corrosion Engineers,Houston, 1974, pp 399-405).

Widespread use of flexible circuits can be found in high volume consumerelectronics and automotive applications, such as stereos. Theseapplications are not constrained by a biological environment. Componentassembly onto flex circuits is commonly achieved by solder attachment.These flex circuits are also much more robust and bulkier than a typicalimplantable device. The standard flex circuit on the market is no lessthan 0.002 inches in total thickness. The trace metalization is etchedcopper foil, rather than thin film metal. Chip-scale package (CSP)assembly onto these flex circuits is done in ball-grid array (BGA)format, which uses solder balls attached to input-output contacts on thepackage base as the interconnect structures. The CSP is aligned to acorresponding metal pad array on the flex circuit and subjected to asolder reflow to create the interconnection. A metallurgicalinterconnect is achieved by solder wetting. The CSP assembly is thenunderfilled with an epoxy material to insulate the solder bumps and toprovide a pre-load force from the shrinkage of the epoxy.

Direct chip attach methods are referred to as chip-on-flex (COF) andchip-on-board (COB). There have been some assemblies that utilize goldwirebonding to interconnect bare, integrated circuits to flexiblecircuits. The flipchip process is becoming a reliable interconnectmethod. Flipchip technology originates from IBM's Controlled CollapseChip Connection (C4) process, which evolved to solder reflow technique.Flipchip enables minimization of the package footprint, saving valuablespace on the circuit, since it does not require a fan out of wirebonds.While there are a variety of flipchip configurations available, solderball attach is the most common method of forming an interconnect. A lessdeveloped approach to flipchip bonding is the use of conductiveadhesive, such as epoxy or polyimide, bumps to replace solder balls.These bumps are typically silver-filled epoxy or polyimide, althoughelectrically conductive particulate of select biocompatible metal, suchas platinum, iridium, titanium, platinum alloys, iridium alloys, ortitanium alloys in dust, flake, or powder form, may alternatively beused. This method does not achieve a metallurgical bond, but relies onadhesion. Polymer bump flip chip also requires underfill encapsulation.Conceivably, polymer bump attachment could be used on a chip scalepackage as well. COB flipchip attach can also be achieved by using goldstud bumps, as an alternative to solder balls. The gold bumps of thechip are bonded to gold contacts on the hard substrate by heat andpressure. A recent development in chip-to-package attachment wasintroduced by Intel Corporation as Bumpless Build Up Layer (BBUL)technology. In this approach, the package is grown (built up) around thedie rather than assembling the die into a pre-made package. BBULpresents numerous advantages in reliability and performance overflipchip.

Known technologies for achieving a bond between a flexible circuit and aelectronics package suffer from biocompatibility issues. Novelapplications of a biomedical implant that utilize a flexible circuitattached to a rigid electronics package require excellentbiocompatibility coupled with long term mechanical attachment stability,to assure long lived reliable electrical interconnection.

Known deposition techniques for a bond, such as an electricallyconductive metal bond or “rivet” are limited to thin layers. Plating isone such known method that does not result in an acceptable bond. It isnot known how to plate shiny platinum in layers greater thanapproximately 1 to 5 microns because the dense platinum layer peels off,probably due to internal stresses. Black platinum lacks the strength tobe a good mechanical attachment, and also lack good electricalconductivity.

Known techniques for bonding an electronic package to a flex circuit donot result in a hermetic package that is suitable for implantation inliving tissue. Therefore, it is desired to have a method of attaching asubstrate to a flexible circuit that ensures that the bonded electronicpackage and flex circuit will function for long-term implantapplications in living tissue.

SUMMARY OF THE INVENTION

An implantable electronic device comprising a hermetic electronicscontrol unit, that is typically mounted on a substrate, that is bondedto a flexible circuit by an electroplated platinum or gold rivet-shapedconnection. The resulting electronics assembly is biocompatible andlong-lived when implanted in living tissue, such as in an eye or ear.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a hermetic, biocompatibleelectronics package that is attached to a flexible circuit.

It is an object of the invention to attach a hermetically sealedelectronics package to a flexible circuit for implantation in livingtissue.

It is an object of the invention to attach a hermetically sealedelectronics package to a flexible circuit for implantation in livingtissue to transmit electrical signals to living tissue, such as theretina.

It is an object of the invention to provide a hermetic, biocompatibleelectronics package that is attached directly to a substrate

It is an object of the invention to provide a method of bonding aflexible circuit to a substrate with an electroplated rivet-shapedconnection.

It is an object of the invention to provide a method of plating platinumas a rivet-shaped connection.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective cutaway view of an eye containing aflexible circuit electrode array.

FIG. 2 is a side view of an electronics package.

FIG. 3 illustrates a cutaway side view of an electronics package.

FIG. 4 is a top view of a flex circuit without the electronics package.

FIG. 5 presents a side view of a flex circuit with the electronicspackage.

FIG. 6 is a side view of a flex circuit that is bonded with adhesive toa hybrid substrate.

FIG. 7 is a series of illustrations of a flexible circuit being bondedusing conductive metal pads to a hybrid substrate.

FIG. 8 is a series of illustrations of weld staple bonding of a flexiblecircuit to a hybrid substrate.

FIG. 9 is a sequence of steps illustrating tail-latch interconnectbonding of a flexible circuit to a hybrid substrate.

FIG. 10 is a sequence of steps illustrating formation of an integratedinterconnect by vapor deposition.

FIG. 11 is a side view of a flexible circuit bonded to a rigid array.

FIG. 12 is a side view of an electronics control unit bonded to anarray.

FIG. 13 is a cross-sectional view of a bonded assembly in stepwisefashion.

FIG. 14 is an electroplating equipment schema.

FIG. 15 is a three-electrode electroplating cell schema.

FIG. 16 is a plot of showing the plating current density decrease withhole size.

FIG. 17 a is a scanning electron micrograph of a polyimide surfacebefore plating magnified 850 times.

FIG. 17 b is a scanning electron micrograph of electrochemicallydeposited rivets magnified 850 times

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for describing the general principlesof the invention. The scope of the invention should be determined withreference to the claims.

The present invention provides a flexible circuit electronics packageand a method of bonding a flexible circuit to a hermetic integratedcircuit which is useful for a number of applications, includingimplantation in living tissue as a neural interface, such as a retinalelectrode array or an electrical sensor. The tissue paper thin flexiblecircuit 18, FIG. 1, transmits electrical signals to the eye 2 by meansof electrodes, that are located in a stimulating electrode array 10,that are in contact with the retina 14. It is obvious that in additionto a stimulating electrode array or sensing electrode, the electrodesmay be contacts connecting to remote electrodes. FIG. 1 illustrates theelectronics control unit 20 in a perspective cutaway view of an eye 2containing a flexible circuit electrode array 18. The electronicscontrol unit 20 is hermetically sealed. The electronics control unit 20may be a hermetic ceramic case with electronics inside, or it may be ahermetically sealed integrated circuit, or any other environmentallysealed electronics package. The stimulating electrode array 10 isimplanted on the retina 14. Flexible circuit ribbon 24 connects thestimulating electrode array 10 to the electronics control unit 20.

The flexible circuit ribbon 24 preferably passes through the sclera 16of the eye 2 at incision 12. Another embodiment of the invention is theflexible circuit ribbon 24 replaced by alternative means of electricalinterconnection, such as fine wires or thin cable. The lens 4 of the eye2 is located opposite the retina 14. A coil 28, which detects electronicsignals such as of images or to charge the electronics control unit 20power supply, located outside the eye 2, near the lens 4, is connectedto the electronics control unit 20 by wire 30.

FIG. 2 illustrates a side view of the hermetic electronics control unit20 and the input/output contacts 22 that are located on the bottom ofthe unit 20. The input/output contacts 22 are bonded in the completedassembly to the flexible circuit 18. Thick film pad 23 is formed byknown thick film technology, such as silk screening or plating.

FIG. 3 illustrates a cutaway side view of the hermetic electronicscontrol unit 20. The pad 23 facilitates attachment of wire 30, and ispreferably comprised of a biocompatible material such as platinum,iridium, or alloys thereof, and is preferably comprised of platinumpaste. Wire 30 is preferably bonded to pad 23 by welding. Themicroelectronics assembly 48 is mounted on the hybrid substrate 44. Vias46 pass through the substrate 44 to input/output contacts 22. Electricalsignals arrive by wire 30 and exit the electronics control unit 20 byinput/output contacts 22.

A top view of the flexible circuit 18 is illustrated in FIG. 4.Electrical signals from the electronics control unit 20 (see FIG. 3)pass into bond pads 32, which are mounted in bond pad end 33. Flexibleelectrically insulating substrate 38, is preferably comprised ofpolyimide. The signals pass from the bond pads 32 along traces 34, whichpass along flexible circuit ribbon 24 to the stimulating electrode array10. The array 10 contains the electrodes 36, which are implanted to makeelectrical contact with the retina 14 of the eye 2, illustrated inFIG. 1. An alternative bed of nails embodiment for the electrodes 36 isdisclosed by Byers, et al. in U.S. Pat. No. 4,837,049.

In FIG. 5, the hermetic electronics control unit 20 is illustratedmounted to flexible circuit 18. In order to assure electrical continuitybetween the electronics control unit 20 and the flexible circuit 18, theelectrical control unit 20 must be intimately bonded to the flexiblecircuit 18 on the bond pad end 33. A cutaway of the electronics controlunit 20 (FIG. 5) illustrates a bonded connection 42. The flexibleelectrically insulating substrate 38 is very thin and flexible and isable to conform to the curvature of the retina 14 (FIG. 1), whenimplanted thereon.

Methods of bonding the flexible insulating substrate 18 to the hermeticelectronics control unit 20 are discussed next.

Platinum Conductor in Polymer Adhesive

A preferred embodiment of the invention, illustrated in FIG. 6, showsthe method of bonding the hybrid substrate 244 to the flexible circuit218 using electrically conductive adhesive 281, such as a polymer, whichmay include polystyrene, epoxy, or polyimide, which containselectrically conductive particulate of select biocompatible metal, suchas platinum, iridium, titanium, platinum alloys, iridium alloys, ortitanium alloys in dust, flake, or powder form.

In FIG. 6, step a, the hybrid substrate 244, which may alternatively bean integrated circuit or electronic array, and the input/output contacts222 are prepared for bonding by placing conductive adhesive 281 on theinput/output contacts 222. The rigid integrated circuit 244 ispreferably comprised of a ceramic, such as alumina or silicon. In stepb, the flexible circuit 218 is preferably prepared for bonding to thehybrid substrate 244 by placing conductive adhesive 281 on bond pads232. Alternatively, the adhesive 281 may be coated with an electricallyconductive biocompatible metal. The flexible circuit 218 contains theflexible electrically insulating substrate 238, which is preferablycomprised of polyimide. The bond pads 232 are preferably comprised of anelectrically conductive material that is biocompatible when implanted inliving tissue, and are preferably platinum or a platinum alloy, such asplatinum-iridium.

FIG. 6, step c illustrates the cross-sectional view A-A of step b. Theconductive adhesive 281 is shown in contact with and resting on the bondpads 232. Step d shows the hybrid substrate 244 in position to be bondedto the flexible circuit 218. The conductive adhesive 281 provides anelectrical path between the input/output contacts 222 and the bond pads232. Step c illustrates the completed bonded assembly wherein theflexible circuit 218 is bonded to the hybrid substrate 144, therebyproviding a path for electrical signals to pass to the living tissuefrom the electronics control unit (not illustrated). The assembly hasbeen electrically isolated and hermetically sealed with adhesiveunderfill 280, which is preferably epoxy.

Studbump Bonding

FIG. 7 illustrates the steps of an alternative embodiment to bond thehybrid substrate 244 to flexible circuit 218 by studbumping the hybridsubstrate 244 and flexible electrically insulating substrate 238 priorto bonding the two components together by a combination of heat and/orpressure, such as ultrasonic energy. In step a, the hybrid substrate 244is prepared for bonding by forming a studbump 260 on the input/outputcontacts 222. The studbump is formed by known methods and is preferablycomprised of an electrically conductive material that is biocompatiblewhen implanted in living tissue if exposed to a saline environment. Itis preferably comprised of metal, preferably biocompatible metal, orgold or of gold alloys. If gold is selected, then it must be protectedwith a water resistant adhesive or underfill 280.

Alternatively, the studbump 260 may be comprised of an insulatingmaterial, such as an adhesive or a polymer, which is coated with anelectrically conductive coating of a material that is biocompatible andstable when implanted in living tissue, while an electric current ispassed through the studbump 260. One such material coating maypreferably be platinum or alloys of platinum, such as platinum-iridium,where the coating may be deposited by vapor deposition, such as byion-beam assisted deposition, or electrochemical means.

FIG. 7, step b presents the flexible circuit 218, which comprises theflexible electrically insulating substrate 238 and bond pads 232. Theflexible circuit 218 is prepared for bonding by the plating bond pads232 with an electrically conductive material that is biocompatible whenimplanted in living tissue, such as with a coating of platinum or aplatinum alloy. Studbumps 260 are then formed on the plated pad 270 byknown methods. Step c illustrates cross-section A-A of step b, whereinthe flexible circuit 218 is ready to be mated with the hybrid substrate244.

FIG. 7, step d illustrates the assembly of hybrid substrate 244 flippedand ready to be bonded to flexible circuit 218. Prior to bonding, thestudbumps 260 on either side may be flattened by known techniques suchas coining. Pressure is applied to urge the mated studbumps 260 togetheras heat is applied to cause the studbumps to bond by a diffusion or amelting process. The bond may preferably be achieved by thermosonic orthermocompression bonding, yielding a strong, electrically conductivebonded connection 242, as illustrated in step e. An example of athermosonic bonding method is ultrasound. The bonded assembly iscompleted by placing an adhesive underfill 280 between the flexiblecircuit 218 and the hybrid substrate 244, also increasing the strengthof the bonded assembly and electrically isolating each bondedconnection. The adhesive underfill 280 is preferably epoxy.

Weld Staple Interconnect

FIG. 8 illustrates the steps of a further alternative embodiment to bondthe hybrid substrate 44 to flexible circuit 18 by weld staple bondingthe substrate 244 and flexible electrically insulating substrate 38together. In step a, a top view of the flexible circuit 18 is shown.Flexible circuit 18 is comprised of flexible electrically insulatingsubstrate 38, which is preferably polyimide, and bond pads 32 having athrough hole 58 therethrough each bond pad 32 and through the top andbottom surfaces of flexible circuit 18. The bond pads 32 are comprisedof an electrically conductive and biocompatible material which is stablewhen implanted in living tissue, and which is preferably platinum or aplatinum alloy, such as platinum-iridium.

FIG. 8, step b presents section A-A, which is shown in the illustrationof step a. The through holes 58 pass completely through each bond pad58, preferably in the center of the bond pad 58. They are preferablyformed by plasma etching. The bond pads 58 are not covered on the topsurface of flexible circuit 18 by flexible electrically insulatingsubstrate 38, thereby creating bond pad voids 56.

FIG. 8, step c shows the side view of hybrid substrate 44 withinput/output contacts 22 on one surface thereof. The hybrid substrate 44is positioned, in step d, to be bonded to the flexible circuit 18 byplacing the parts together such that the input/output contacts 22 arealigned with the bond pads 32. Then wire 52, which is preferably a wire,but may equally well be a ribbon or sheet of weldable material, that isalso preferably electrically conductive and biocompatible when implantedin living tissue, is attached to input/output contact 22 and bond pad 32to bond each aligned pair together. The wire 52 is preferably comprisedof platinum, or alloys of platinum, such as platinum-iridium. The bondis preferably formed by welding using the parallel gap welder 50, whichmoves up and down to force the wire 52 into the through hole 58 and intocontact with input/output contact 22. This process is repeated for eachaligned set of input/output contacts 22 and bond pads 32, as shown instep e.

The weld staple interconnect bonding process is completed, as shown instep f, by cutting the wire 54, leaving each aligned set of input/outputcontacts 22 and bond pads 32 electrically connected and mechanicallybonded together by staple 54.

Tail-Latch Interconnect

FIG. 9 illustrates yet another embodiment for attaching the hybridsubstrate 244 to a flexible circuit 218 by using a tail-ball 282component, as shown in step a. The hybrid substrate 244 is preferablycomprised of a ceramic material, such as alumina or silicon. In oneembodiment, a wire, preferably made of platinum or another electricallyconductive, biocompatible material, is fabricated to have a ball on oneend, like the preferred tail-ball 282 illustrated in step a. Thetail-ball 282 has tail 284 attached thereto, as shown in the side viewof step a. The tail-ball 282 is aligned with input/output contact 222 onhybrid substrate 244, in preparation to being bonded to flexible circuit218, illustrated in step b.

The top view of step b illustrates flexible electrically insulatingsubstrate 238, which is preferably comprised of polyimide, having thethrough hole 237 passing completely thorough the thickness and alignedwith the tail 284. The bond pads 232 are exposed on both the top andbottom surfaces of the flexible circuit 218, by voids 234, enablingelectrical contact to be made with input/output contacts 222 of thehybrid substrate 244. The voids are preferably formed by plasma etching.

The side view of FIG. 9, step c, which illustrates section A-A of stepb, shows the hybrid substrate 244 in position to be bonded to andaligned with flexible circuit 218. The tails 284 are each placed inthrough hole 237. Pressure is applied and the tail-balls 282 are placedin intimate contact with bond pads 232 and input/output contacts 222.Step c illustrates that each of the tails 284 is bent to make contactwith the bond pads 232. The bonding process is completed by bonding,preferably by welding, each of the tails 284, bond pads 232, tail-balls282, and input/output contacts 222 together, thus forming a mechanicaland electrical bond. Locking wire 262 is an optional addition to assurethat physical contact is achieved in the bonded component. The processis completed by underfilling the gap with an electrically insulating andbiocompatible material (not illustrated), such as epoxy.

Integrated Interconnect by Vapor Deposition

FIG. 10 illustrates a further alternative embodiment to creating aflexible circuit that is electrically and adhesively bonded to ahermetic rigid electronics package. In this approach, the flexiblecircuit is fabricated directly on the rigid substrate. Step a shows thehybrid substrate 44, which is preferably a ceramic, such as alumina orsilicon, having a total thickness of about 0.012 inches, with patternedvias 46 therethrough. The vias 46 are preferably comprised of fritcontaining platinum.

In step b, the routing 35 is patterned on one side of the hybridsubstrate 44 by known techniques, such as photolithography or maskeddeposition. It is equally possible to form routing 35 on both sides ofthe substrate 44. The hybrid substrate 44 has an inside surface 45 andan outside surface 49. The routing 35 will carry electrical signals fromthe integrated circuit, that is to be added, to the vias 46, andultimately will stimulate the retina (not illustrated). The routing 35is patterned by know processes, such as by masking during deposition orby post-deposition photolithography. The routing 35 is comprised of abiocompatible, electrically conductive, pattemable material, such atplatinum.

Step c illustrates formation of the release coat 47 on the outsidesurface 49 of the hybrid substrate 44. The release coat 47 is depositedby known techniques, such as physical vapor deposition. The release coat47 is removable by know processes such as etching. It is preferablycomprised of an etchable material, such as aluminum.

Step d illustrates the formation of the traces 34 on the outside surface49 of the hybrid substrate 44. The traces 34 are deposited by a knownprocess, such as physical vapor deposition or ion-beam assisteddeposition. They may be patterned by a known process, such as by maskingduring deposition or by post-deposition photolithography. The traces 34are comprised of an electrically conductive, biocompatible material,such as platinum, platinum alloys, such as platinum-iridium, ortitanium-platinum. The traces 34 conduct electrical signals along theflexible circuit 18 and to the stimulating electrode array 10, whichwere previously discussed and are illustrated in FIG. 4.

Step e illustrates formation of the flexible electrically insulatingsubstrate 38 by known techniques, preferably liquid precursor spinning.The flexible electrically insulating substrate 38 is preferablycomprised of polyimide. The flexible electrically insulating substrateelectrically insulates the traces 34. It is also biocompatible whenimplanted in living tissue. The coating is about 5 um thick. The liquidprecursor is spun coated over the traces 34 and the entire outsidesurface 49 of the hybrid substrate 44, thereby forming the flexibleelectrically insulating substrate 38. The spun coating is cured by knowntechniques.

Step f illustrates the formation of voids in the flexible electricallyinsulating substrate 38 thereby revealing the traces 34. The flexibleelectrically insulating substrate is preferably patterned by knowntechniques, such as photolithography with etching.

Step g illustrates the rivets 51 having been formed over and in intimatecontact with traces 34. The rivets 51 are formed by known processes, andare preferably formed by electrochemical deposition of a biocompatible,electrically conductive material, such as platinum or platinum alloys,such as platinum-iridium.

Step h illustrates formation of the metal layer 53 over the rivets 51 ina controlled pattern, preferably by photolithographic methods, on theoutside surface 49. The rivets 51 and the metal layer 53 are in intimateelectrical contact. The metal layer 53 may be deposited by knowntechniques, such as physical vapor deposition, over the entire surfacefollowed by photolithographic patterning, or it may be deposited bymasked deposition. The metal layer 53 is formed of an electricallyconductive, biocompatible material, which in a preferred embodiment isplatinum. The patterned metal layer 53 forms traces 34 and electrodes36, which conduct electrical signals from the electronics control unit20 and the electrodes 36 (see FIGS. 4 and 5).

Step i illustrates the flexible electrically insulating substrate 38applied over the outside surface 49 of the rigid substrate 44, as instep e. The flexible electrically insulating substrate 38 covers therivets 51 and the metal layer 53.

Step j illustrates the hybrid substrate 44 having been cut by knownmeans, preferably by a laser or, in an alternative embodiment, by adiamond wheel, thereby creating cut 55. The portion of hybrid substrate44 that will be removed is called the carrier 60.

The flexible electrically insulating substrate 38 is patterned by knownmethods, such as photolithographic patterning, or it may be deposited bymasked deposition, to yield voids that define the electrodes 36. Theelectrodes 36 transmit electrical signals directly to the retina of theimplanted eye (see FIG. 4).

Step k illustrates flexible circuit 18 attached to the hybrid substrate44. The carrier 60 is removed by utilizing release coat 47. In apreferred embodiment, release coat 47 is etched by known means torelease carrier 60, leaving behind flexible circuit 18.

Step 1 illustrates the implantable electronic device of a flexiblecircuit 18 and an intimately bonded hermetic electronics control unit20. The electronics control unit 20, which contains the microelectronicsassembly 48, is hermetically sealed with header 62 bonded to rigidcircuit substrate 44. The header 62 is comprised of a material that isbiocompatible when implanted in living tissue and that is capable ofbeing hermetically sealed to protect the integrated circuit electronicsfrom the environment.

FIG. 11 illustrates an electronics control unit 320 attached to flexibleelectrically insulating substrate 338, which is preferably comprised ofpolyimide, by bonded connections 342. The electronics control unit 320is preferably a hermetically sealed integrated circuit, although in analternative embodiment it may be a hermetically sealed hybrid assembly.Bonded connections 342 are preferably conductive adhesive, although theymay alternatively be solder bumps. The bond area is underfilled with anadhesive 380. Rigid stimulating electrode array 310 is attached to theflexible electrically insulating substrate 338 by bonded connections342.

FIG. 12 illustrates an electronics control unit 320 attached to rigidstimulating electrode array 310 by bonded connections 342. The bond areais then underfilled with an adhesive 380, preferably epoxy. Bondedconnections 342 are preferably conductive adhesive, although they mayalternatively be solder bumps.

The bonding steps are illustrated in FIG. 13 for a flex circuit assemblythat is bonded with rivets 61 that are created in situ by a depositionprocess, preferably by electroplating. The rivets 61 are rivet-shapedelectrical connections. The substrate 68 is shown generally in FIG. 13.It is comprised of the hybrid substrate 44, which is preferably aceramic, such as alumina or silicon. The silicon would preferably becoated with a biocompatible material to achieve biocompatibility of thesilicon, which is well known to slowly dissolve when implanted in livingtissue.

The hybrid substrate 44 preferably contains vias 46 that pass throughthe thickness of the hybrid substrate 44, see FIG. 13, step (a). Vias 46are not required to enable this invention, and are not present inalternative embodiments. It is preferred that the hybrid substrate 44 berigid, although alternative embodiments utilize a non-rigid substrate.The vias 46 are integral with electrically conductive routing 35 thathas been placed on the surface of the hybrid substrate 44 by knowntechniques. The routing is preferably comprised of a stablebiocompatible material, such as platinum, a platinum alloy, or gold,most preferably platinum.

A flexible electrically insulating substrate 38 is preferably comprisedof two layers of an electrically insulating material, such as a polymer.Known preferred polymer materials are polyimide or Parylene. Parylenerefers to polyparaxylylene, a known polymer that has excellent implantcharacteristics. For example, Parylene, manufactured by SpecialtyCoating Systems (SCS), a division of Cookson Electronic Equipment Group,located in Indianapolis, Ind., is a preferred material. Parylene isavailable in various forms, such as Parylene C, Parylene D, and ParyleneN, each having different properties. The preferred form is Parylene C.

The flexible electrically insulating substrate layers 38 are preferablyof approximately equal thicknesses, as illustrated in FIG. 13, step (a).A trace 65 is also illustrated in FIG. 13, step (a), where trace 65 maybe at least one, but preferably more than one, trace 65 that iselectrically conductive. The traces 65 are integrally bonded to bondpads 63. The bond pads 63 each have a bond pad hole 64 therethrough,which is in approximate alignment with first hole 57 in firstelectrically insulating substrate 37 and second hole 59 in the secondflexible electrically insulating substrates 38, such that there is ahole, with centers approximately aligned, through the thickness of theflexible assembly 66.

The flexible assembly 66 is placed next to the hybrid substrate inpreparation for bonding, FIG. 13, step (b). The flexible assemblyaligned holes that are formed by first substrate holes 57, bond padholes 64, and second substrate holes 59 are aligned with the routing 35.In a preferred embodiment, there is at least one via 46, although no via46 is required. In a preferred embodiment, an adhesive layer 39 isapplied to adhesively bond the assembly together. The adhesive ispreferably epoxy, silicone, or polyimide. In alternative embodiments,the assembly is not adhesively bonded.

As illustrated in FIG. 13, step (c), a rivet 61 is formed in eachflexible substrate hole to bond the assembly together. The rivets 61 arepreferably formed by a deposition process, most preferablyelectroplating. The rivets 61 are comprised of a biocompatible,electrically conductive material, preferably platinum, althoughalternative embodiments may utilize platinum alloys (e.g.platinum-iridium or platinum-rhodium), iridium, gold, palladium, orpalladium alloys. It is most preferred that rivet 61 be comprised ofelectroplated platinum, called “plated platinum” herein.

Referring to FIGS. 14 and 15, a method to produce plated platinumaccording to the present invention is described comprising connecting acommon electrode 402, the anode, and a bonded assembly 70, the cathode,to a voltage to current converter 406 with a wave form generator 430 andmonitor 428, preferably an oscilloscope. The common electrode 402,bonded assembly 70, a reference electrode 410, for use as a reference incontrolling the power source, which is comprised of a voltage to currentconverter 406 and a waveform generator 430, and an electroplatingsolution are placed in a electroplating cell 400 having a means formixing 414 the electroplating solution. Power may be supplied to theelectrodes with constant voltage, constant current, pulsed voltage,scanned voltage or pulsed current to drive the electroplating process.The waveform generator 430 and voltage to current converter 406 is setsuch that the rate of deposition will cause the platinum to deposit asplated platinum of the present invention, the rate being greater thanthe deposition rate necessary to form shiny platinum and less than thedeposition rate necessary to form platinum black.

Because no impurities or other additives, such as lead, which is aneurotoxin and cannot be used in an implantable device, need to beintroduced during the plating process to produce plated platinum of thepresent invention, the plated material can be pure platinum.Alternatively, other materials can be introduced during the platingprocess, if so desired, but these materials are not necessary to theformation of plated platinum of the present invention.

Referring to FIGS. 14 and 15, the electroplating cell 400, is preferablya 50 ml to 150 ml four neck glass flask or beaker, the common electrode402, or anode, is preferably a large surface area platinum wire orplatinum sheet, the reference electrode 410 is preferably a Ag/AgClelectrode (silver, silver chloride electrode), the bonded assembly 70,or cathode, can be any suitable material depending on the applicationand can be readily chosen by one skilled in the art. Preferable examplesof the bonded assembly 70 include, but are not limited to, platinum,iridium, rhodium, gold, tantalum, titanium or niobium, preferablyplatinum.

The means for mixing 414 is preferably a magnetic stirrer (FIG. 15). Theplating solution is preferably 3 to 30 millimoles ammoniumhexachloroplatinate in 0.4 moles of disodium hydrogen phosphate, but maybe derived from any chloroplatinic acid or bromoplatinic acid or otherelectroplating solution. The preferable plating temperature isapproximately 24° C.-26° C.

The electroplating system for pulsed current control is shown in FIGS.14 and 15. While constant voltage, constant current, pulsed voltage orpulsed current can be used to control the electroplating process, pulsedcurrent control of the plating process is preferable for plating rivets61, which have a height that approximates their diameter. The preferablecurrent range to produce plated platinum of the present invention, whichvaries from about 50 to 2000 mA/cm², is dependent on the holedimensions, FIG. 16, where the response voltage ranges from about −0.45volts to −0.85 volts. Applying power in this range with the abovesolution yields a plating rate in the range of about 0.05 um per minuteto 1.0 um per minute, the preferred range for the plating rate of platedplatinum of the present invention. The average current density may bedetermined by the equation y=19572x^(−1.46), where y is the averagecurrent density in mA/cm² and x is the hole diameter in microns. Pulsedcurrent control also allows an array of rivets to be platedsimultaneously achieving uniform rivet properties.

As plating conditions, including but not limited to the platingsolution, surface area of the electrodes, pH, platinum concentration andthe presence of additives, are changed the optimal control parameterswill change according to basic electroplating principles. Platedplatinum of the present invention will be formed so long as the rate ofdeposition of the platinum particles is slower than that for theformation of platinum gray and faster than that for the formation ofshiny platinum.

It has been found that because of the physical strength of platedplatinum of the present invention, it is possible to plate rivets ofthickness greater than 30 microns. It is very difficult to plate shinyplatinum in layers greater than approximately two microns because theinternal stresses of the dense platinum layer cause the plated layer topeel off.

Plated platinum of the present invention can be distinguished from twoother forms of electroplated platinum, specifically, platinum gray andplatinum black, based on the adhesive strength of a thin film coating ofthese materials. Adhesive strength of thin film coatings of platinumgray and platinum black on electrically conductive articles has beenmeasured on a Micro-Scratch Tester (CSEM Instruments, Switzerland). A400 micron micro-scratch is formed by drawing a diamond tip, having a 10micron spherical radius, across the coating under a load that isincreased in a controlled manner from 1 millinewton to 100 millinewtons.At the “critical load”, the coating fails. Using this test, it is foundthat plated platinum of the present invention fails at an estimatedcritical load of about 70 millinewtons, platinum gray fails at ameasured critical load of 61 millinewtons, while platinum black fails ata measured critical load of less than about 35 millinewtons.

Likewise, the several forms of platinum are distinguished by theirhardness. For example, the microhardness, as measured with a Vickersindenter, is estimated to be about 30 for plated platinum of the presentinvention, while it is estimated to be less than one for platinum black,it is measured to be 17 for platinum gray, and is estimated to be about34 for shiny platinum, and it is reported to be 40 for platinum foil,and 100 for cold worked platinum (Materials Engineering, MaterialsSelector 1990, Penton Publishing, 1989, p 122). On a hybrid substrate44, a thin-layer routing 35, preferably platinum, is sputtered and thencovered with about 6 μm thick flexible assembly 66, preferablypolyimide, with holes in the range from 5 μm to 50 μm. On each sample,preferably about 100 to 700 or more such holes are exposed for platingof rivets 61, see FIG. 17 a.

SEM micrographs record the surface appearance before plating. Thesurface is chemically and electrochemically cleaned before plating.

The electrodes in the test cell are arranged, so that the bondedassembly 70 (cathode) is physically parallel with the common electrode402 (anode). The reference electrode 410 is positioned beside the bondedassembly 70. The plating solution is added to electroplating solutionlevel 411. The solution is comprised of about 18 millimoles ammoniumhexachloroplatinate in about 0.4 moles phosphate buffer solution. Theamount of solution used depends on the number of rivets 61 to be plated.The means for mixing 414, preferably a magnetic stirrer, is activated.

A voltage waveform is generated, preferably with a 1 msec pulse width asa 500 Hz square wave, which is converted to a current signal through avoltage to current converter 406.

The pulse current is applied to the plating electrode versus anode. Theelectrode voltage versus Ag/AgCl reference electrode is monitored usingan oscilloscope (Tektronix TDS220 Oscilloscope). The current amplitudeis adjusted so that the cathodic peak voltage reaches about −0.6 Vversus the Ag/AgCl reference electrode 410. During plating, theelectrode voltage tends to decrease with plating time. The currentamplitude is frequently adjusted so that the electrode voltage is keptwithin −0.5 to −0.7 V range versus Ag/AgCl reference electrode 410. Whenthe specified plating time is reached, the current is eliminated. Thecathode is rinsed in deionized water thoroughly. Typical plating time isin the range of about 5 to 60 minutes, preferably 15 to 25 minutes.

The plated surface is examined under an optical microscope. Opticalphotomicrographs are taken at both low and high magnifications to recordthe image of the surface. The plated samples are profiled with a surfaceprofilometer to measure the dimensions of the plated rivet. The totalplated rivet has a total height of about 8 to 16 μm.

After plating, the pulsing current amplitudes are averaged for the totalplating time and recorded. It has been demonstrated that the currentdensity increases exponentially with sample hole decrease. The smallerthe sample holes, the higher the current density required (see FIG. 16).

An illustrative example of a plated platinum rivet according to thepresent invention are micrographs produced on a Scanning ElectronMicroscope (SEM) at 850.times. taken by a JEOL JSM5910 microscope, FIGS.17 a and 17 b.

Furthermore, it has been found that because of the physical strength ofplated platinum of the present invention, it is possible to plate rivets61 of thickness greater than 16 um. It is very difficult to plate shinyplatinum in layers greater than approximately 1 to 5 μm because theinternal stress of the dense platinum layer which will cause platedlayer to peel off.

The following example is illustrative of electroplating platinum as arivet 61, according to the present invention.

EXAMPLE

A flexible electrically insulating substrate comprised of a firstsubstrate 37 and a second substrate 38 of polyimide having a totalthickness of 6 um. It had 700 first substrate holes 57, an equal numberof matching bond pad holes 64, and an equal number of matching secondsubstrate holes 59, all in alignment so as to create a continuous holethrough flexible assembly 66 that terminates on routing 35, arranged in100 groups of seven on about 40 um centers, FIG. 4 a. The hybridsubstrate 44 was alumina and the routing 35 was platinum. The bond pad63 was platinum.

The assembly was cleaned by rinsing three times in 10% HCl. It wasfturther prepared by bubbling for 10 seconds at +/−5V at 1 Hz inphosphate buffered saline Finally, it was rinsed in deionized water.

The electroplating set up according to FIGS. 14 and 15 was comprised ofan electroplating cell 400 that was a 100 ml beaker with anelectroplating solution level 411 at about the 75 ml level. The solutionwas 18 millimoles of ammonium hexachloroplatinate in 0.4 moles phosphatebuffer solution.

The means for mixing 414 was a magnetic stirrer, which was activated.The voltage waveform of 1 msec pulse width as a square wave, wasgenerated by an HP 33120A waveform generator, which is converted tocurrent signal through a voltage to current converter 406. The pulsecurrent was 1 msec in pulse width at 500 Hz square wave.

The pulse current was applied on the plating electrode bonded assembly70 versus common electrode 402. The electrode voltage versus Ag/AgClreference electrode 410 was monitored using as a monitor 428 a Tektronixmodel TDS220 oscilloscope. The current amplitude was increased so thatthe bonded assembly 70 (cathode) peak voltage reached −0.6 V versus theAg/AgCl reference electrode 410. During plating, the electrode voltagedecreased with plating time.

The average current density was 660 mA/cm², which generated responsevoltages of −0.5 to −0.7 volts, where the voltage was controlled by thecurrent. A 1 msec pulse width square wave was generated by an HP 33120AArbitrary Waveform Generator. The pulse was converted to a currentsignal through a voltage to current converter 406. The pulse current wastypically about 1 msec in pulse width as a 500 Hz square wave. Theresulting plated platinum rivet 61 was about 32 μm diameter on thebutton end and about 15 μm tall, with about 9 μm of the height extendingabove the polyimide substrate. The plated platinum rivet was dense,strong, and electrically conductive.

Scanning Electron Microscope (SEM)/energy dispersive analysis (EDAX.TM.)analysis were performed on the rivets 61. SEM micrographs of the platedsurface were taken showing its as-plated surface, FIG. 17 b. Energydispersed analysis demonstrated that the rivet 61 was pure platinum,with no detectable oxygen.

The above described is the preferred embodiment of the currentinvention, however the platinum electrodeposition described in U.S. Pat.No. 6,974,533 and incorporated herein by reference, is also effectivefor forming electrochemically deposited rivets.

The rivet 61 (FIG. 13) forms an electrically conductive bond with therouting 35 and with the bond pad 63. It is obvious that the bondedassembly may be stacked with other bonded assemblies forming multiplestacked assemblies with increased stacking density.

Accordingly, what has been shown is an improved flexible circuit with anelectronics control unit attached thereto, which is suitable forimplantation in living tissue and to transmit electrical impulses to theliving tissue. Obviously, many modifications and variations of thepresent invention are possible in light of the above teachings. It istherefore to be understood that, within the scope of the appendedclaims, the invention may be practiced other than as specificallydescribed.

1. A device comprising: a substrate at least a portion of which iselectrically conductive; a flexible assembly; and at least one depositedrivet that bonds said substrate and said flexible assembly together. 2.The device according to claim 1 wherein said device is implantable. 3.The device according to claim 1 wherein said at least one depositedrivet is deposited electrochemically.
 4. The device according to claim 1wherein a biocompatible adhesive bonds said substrate and said flexibleassembly.
 5. The device according to claim 1 wherein said rivetcomprises a biocompatible material.
 6. The device according to claim 1wherein said rivet comprises platinum or a platinum alloy.
 7. The deviceaccording to claim 6 wherein electrochemical deposition is accomplishedat a current density of 50 to 2000 mA/cm².
 8. The device according toclaim 6 wherein electrochemical deposition is accomplished by pulsecurrent control.
 9. The device according to claim 1 wherein saidflexible assembly comprises polyimide.
 10. The device according to claim1 wherein said substrate comprises a biocompatible ceramic.
 11. Thedevice according to claim 10 wherein said biocompatible ceramiccomprises alumina.
 12. The device according to claim 1 wherein: at leasta part of said substrate forms an electronics control unit; and saidelectronics control unit encapsulates electronics.
 13. The deviceaccording to claim 1 wherein said substrate is rigid.
 14. The deviceaccording to claim 1 wherein said substrate is an electrically insulatedintegrated circuit.
 15. The device according to claim 1 wherein saidflexible assembly is a thin integrated circuit.
 16. A method of making adevice according to claim 1 comprising the steps of: providing asubstrate at least a portion of which is electrically conductive; andelectrochemically depositing a layer of platinum greater than one micronthick, with a Vickers hardness greater than ten, on said electricallyconductive portion.
 17. The method of claim 16 further comprising thestep of selecting a charge density for electrochemically depositing thatis proportional to the area of said electrically conductive portion. 18.The method of claim 16 wherein said electrochemically deposited layer isgreater than ten microns thick.
 19. A method of bonding for implantationa substrate to a flexible assembly, said method comprising the steps of:aligning said substrate and said flexible assembly, whereby there is acommon alignment for a routing on said substrate and a through holedefined by said flexible assembly; and depositing an electricallyconductive rivet in said through hole, thereby bonding said flexibleassembly to said substrate.
 20. The method according to claim 19 furthercomprising the step of applying an electrically non-conductive adhesivebetween said substrate and said flexible assembly.